Clear handler reference on OcAgentTraceExporter unregister (#1965)

This allows the exporter to be re-registered without triggering an
IllegalStateException.

Author: c2nes

exporters/trace/ocagent/src/main/java/io/opencensus/exporter/trace/ocagent/OcAgentTraceExporter.java

@@ -108,7 +108,10 @@ private static void registerInternal(Handler newHandler) {
    * @since 0.20
    */
   public static void unregister() {
-    unregister(Tracing.getExportComponent().getSpanExporter());
+    synchronized (monitor) {
+      unregister(Tracing.getExportComponent().getSpanExporter());
+      handler = null;
+    }
   }
 
   /**